Photoconductive co-crystalline complex of pyrylium dye and polymer used in electrophotography

ABSTRACT

A METHOD IS PROVIDED FOR FORMING A CO-CRYSTALLINE COMPLEX OF PYRYLIUM DYE AND POLYMER WHICH INVOLVES DISSOLVING THE COMPONENTS IN A SOLVENT SYSTEM IN WHICH THE SOLUBILITIES OF THE COMPONENTS ARE SUB-STANTIALLY EQUAL. A NONPOLAR PRECIPITATING LIQUID IN WHICH THE COMPONENTS ARE INSOLUBLE IS COMBINED WITH THE SOLTUION ABOVE TO PRODUCE PRECIPITATION OF THE COMPLEX WHICH IS THEN REMOVED FROM THE VARIOUS LIQUIDS. THE RESULTANT COMPLEX IS PHOTOCONDUCTIVE AND CAN BE USED TO SENSITIZE OTHER PHOTOCONDUCTORS. ELECTROPHOTOGRAPHIC ELEMENTS ARE PREPARED BY DISPERSING THE SO-FORMED COMPLEX IN A POLYMERIC VEHICLE AND COATING THE DISPERSION ON A SUPPORT.

United States Patent 3,684,502 PHOTOCONDUCTIVE CO-CRYSTALLINE COM- PLEX0F PYRYLIUM DYE AND POLYMER USED IN ELECTROPHOTOGRAPHY Eugene P. Gramzaand David D. Schreiber, both of Kodak Park, 1669 Lake Ave., Rochester,N.Y. 14615 No Drawing. Filed Nov. 18, 1970, Ser. No. 90,778

Int. Cl. G03g 5/04 U.S. Cl. 961.6 9 Claims ABSTRACT OF THE DISCLOSUREThis invention relates to electrophotography and to photoconductiveelements and structures useful in electrophotography. In addition, thisinvention relates to methods for preparing electrophotographic elements.

Electrophotographic imaging processes and techniques have beenextensively described in both the patent and other literature.

Generally, these processes have in common the steps of employing anormally insulating photoconductive element which is prepared to respondto imagewise exposure with electromagnetic radiation by forming a latentelectrostatic charge image. A variety of subsequent operations, now wellknown in the art, can then be employed to produce a permanent record ofthe image.

One type of photoconductive insulating structure or element particularlyuseful in electrophotography utilizes a composition containing aphotoconductive insulating material dispersed in a resinous material. Aunitary electrophotographic element is generally produced in amultilayer type of structure by coating a layer of the photoconductivecomposition onto a film support previously overcoated with a layer ofconducting material or the photoconductive composition may be coateddirectly onto a conducting support of metal or other suitable conductingmaterial. Such photoconductive compositions have shown improved speedand/or spectral response, as well as other desired electrophotographiccharacteristics when one or more photosensitizing materials or addendaare incorporated into the photoconductive composition. Typical addendaof this latter type are disclosed in US. Pat. Nos. 3,250,615, issued May10, 1966, by Van Allan; 3,141,770, issued July 21, 1964, by Davis etal.; and 2,987,395, issued June 6, 1961, by Jarvis. Generally,photosensitizing addenda to photoconductive compositions areincorporated to etfect a change in the sensitivity or speed of aparticular photoconductor system and/or a change in its spectralresponse characteristics. Such addenda can enhance the sensitivity of anelement to radiation at a particular wavelength or to a broad range ofwavelengths Where desired. The mechanism of such sensitization ispresently not fully understood. The phenomenon, however, is extremelyuseful. The importance of such effects is evidenced by the extensivesearch currently conducted by workers in the art for compositions andcompounds which are capable of photosensitiz- 3,684,502 Patented Aug.15, 1972 c CC ing photoconductive compositions in the manner described.

Usually, the desirability of a change in electrophotographic propertiesis dictated by the end use contemplated for the photoconductive element.For example, in document copying applications, the spectralelectrophotographic response of the photoconductor should be capable ofreproducing the Wide range of colors which are normally encountered insuch use. If the response of the photocnductor falls short of thesedesign criteria, it is highly desirable if the spectral response of thecomposition can be altered by the addition of photosensitizing addendato the composition. Likewise, various applications specifically requireother characteristics such as the ability of the element to accept ahigh surface potential, and exhibit a low dark decay of electricalcharge. It is also desirable for the photoconductive element to exhibithigh speed as measured in an electrical speed or characteristic curve, alow residual potential after exposure and resistance to fatigue.Sensitization of many photoconductive compositions by the addition ofcertain dyes selected from the large number of dyes presently known hashitherto been widely used to provide for the desired flexibility in thedesign of photoconductive elements in particularphotoconductor-containing systems. Conventional dye addenda tophotoconductor compositions have generally shown only a limitedcapability for over-all improvement in the totality ofelectrophotographic properties which cooperate to produce a usefulelectrophotographic element or structure. The art is still searching forimprovements in shoulder and toe speeds, improved solid areareproduction characteristics, rapid recovery and usefulelectrophotographic speed from either positive or negative electrostaticcharging.

A high speed heterogeneous or aggregate photoconductive system wasdeveloped by William A. Light which overcomes many of the problems ofthe prior art. This aggreate composition is the subject matter ofcopending application Ser. No. 804,266, filed Mar. 4, 1969, now U.S.Pat. No. 3,615,414, and entitled Novel Photoconductive Compositions andElements. The addenda disclosed therein are responsible for theexhibition of desirable electrophotographic properties inphotoconductive elements prepared therewith. However, in accordance withthe procedures described therein, the preparation of electrophotographicelements uses a solvent treatment step subsequent to the coating step.In an effort to avoid this secondary treatment step, a novel method ofpreparation of photoconductive compositions of the type described byLight is disclosed in copending Eugene P. Gramza Application Ser. No.821,513, filed May 2, 1969, now U.S. Pat. No. 3,615,415, and entitledMethod for the Preparation of Photoconductive Compositions. This lattermethod involves the high speed shearing of the photoconductivecomposition prior to coating and thus eliminates subsequent solventtreatment. However, it is often desirable to have photoconductivecompositions of even higher speeds than those obtainable with the abovecompositions. Thus, copending Edward J. Seus application Ser. No.764,302, filed Oct. 1, 1968, now U.S. Pat. No. 3,591,374, and entitledHigh Speed Electrophotographic Elements and Method for PreparationThereof discloses a technique for substantially increasing the speed ofthe above compositions. This technique involves formingelectrophotographic layers by the above techniques'and then overcoatingsuch layers with a solution of suitable dye. This latter procedure usesa secondary coating step. Accordingly, there is a need situ in a bindercomposition which may or may not be that desired for the ultimate use ofthe electrophotographic element thus formed. Accordingly, there is aneed for a method of forming isolated aggregate compositions which canbe dispersed as addenda in a variety of photoconductive compositions.

It is, therefore, an object of this invention to provide the art ofelectrography with a novel method of preparing photoconductivecompositions.

It is an additional object of this invention to provide novel isolateddye-polymer aggregates which can subsequently be dispersed in apolymeric binder to form a photoconductive composition and a novelmethod for their preparation.

It is another object to provide a novel process for formingelectrophotographic elements.

These and other objects and advantages will become apparent from thefollowing description of the invention.

It has been discovered that when the components of the compositionsdescribed by Light are manipulated in a certain prescribed manner,aggregates are formed which can be isolated and then be dispersed in anydesired binder system. This technique allows extended latitude in theuse of these aggregates in that it is no longer necessary to form theaggregates in situ in a particular binder system. This technique thusprovides greater freedom in selecting a binder system desired andprovides a simplified means of forming electrophotographic elementswithout any required secondary treatment. This method involvesdissolving the polymer and dye components of the aggregate in ahalogenated solvent system in which the solubilities of the twoingredients are substantially equal. A precipitating liquid is thenadded in which the aggregate is insoluble. The addition of theprecipitating liquid causes the aggregate compositions to precipitate.The solvent and precipitating liquid are separated from the precipitatewhich is then dried.

The aggregate crystals are now ready to be dispersed in a suitablebinder system which can contain any desired photoconductor. Aggregatecrystals are added to a solution of binder and photoconductor in asolvent which has substantially no solvent action on the aggregate. Thecomposition is mixed by any suitable means and coated using standardcoating techniques. After drying, the resultant layer comprises amultiphase composition, the heterogeneous nature of which is generallyapparent when viewed under 2500 magnification, although suchcompositions may appear to be substantially optically clear to the nakedeye in the absence of magnification. There can, of course, be amacroscopic heterogeneity. Suitably, the dye-containing aggregate in thediscontinuous phase is predominantly in the size range of from about0.01 to microns. However, it should be noted that when the aggregatecrystals prepared according to the invention are use to sensitize aparticulate photoconductor, such as zinc oxide, another discontinuousphase will be present which may not fall within this size range.

In general, the heterogeneous compositions formed, as described above,are multiphase organic solids containing dye and polymeric vehicle. Thepolymeric vehicle forms an amorphous matrix or continuous phase whichcontains a discrete discontinuous phase as distinguished from asolution. The discontinuous phase is comprised of aggregate crystalsprepared by the instant precipitation technique and are comprised of aco-crystalline complex of dye and polymer. When aggregate crystalsprepared as described herein are used in conjunction with an organicphotoconductor, the resultant photoconductive composition generallycontains only two phases as the photoconductor usually forms a solidsolution with the continuous phase of polymeric vehicle. On the otherhand, when a particulate photoconductor, such as as zinc oxide, is usedthree phases may be present. In such a case, there would be a continuouspolymeric phase, a discontinuous phase containing aggregate as discussedabove and another discontinuous phase comprised of the particulatephotoconductor. Of course, such multiphase compositions may also containadditional discontinuous phases of trapped impurities, etc. Anotherfeature characteristic of the heterogeneous compositions formed asdescribed above is that the wavelength of the radiation absorptionmaximum characteristic of such compositions is substantially shiftedfrom the wavelength of the radiation absorption maximum of asubstantially homogeneous solid solution formed of similar constituents.Such an absorption maximum shift in the formation of multiphaseheterogeneous systems for the present invention is generally of themagnitude of at least about 10 nm. If mixtures of dyes are used, one dyemay cause an absorption maximum shift to a long wavelength and anotherdye cause an absorption maximum to a shorter wavelength.

Sensitizing dyes and electrically insulating polymeric materials areused in forming the aggregate compositions. Typically, pyrylium dyes,including pyrylium, thiapyrylium and selenapyrylium dye salts are usefulin forming such. compositions. Such dyes include those which can berepresented by the following general formula:

Raxi Re wherein R, R, R, R and R can each represent (a) a hydrogen atom;(b) an alkyl group typically having from 1 to 15 carbon atoms, such asmethyl, ethyl, propyl, isopropyl, butyl, tertiary butyl, amyl, isoamyl,hexyl, octyl, nonyl, dodecyl, etc., (c) alkoxy groups like methoxy,ethoxy, propoxy, butoxy, amyloxy, hexoxy, octoxy, and the like; and (d)aryl groups including substituted aryl groups such as phenyl,4-diphenyl, alkylphenyls as 4- ethylphenyl, 4-propylphenyl, and thelike, alkoxyphenyls as 4-ethoxyphenyl, 4-methoxyphenyl, 4-amyloxyphenyl,2- hexoxyphenyl, 2-methoxyphenyl, 3,4-dimethoxyphenyl, and the like,fi-hydroxy alkoxyphenyls as Z-hydroxyethoxyphenyl,3-hydroxyethoxyphenyl, and the like, 4- hydroxyphenyl, halophenyls as2,4-dichlorophenyl, 3,4-dibrornophenyl, 4-chlorophenyl,3,4-dichlorophenyl, and the like, azidophenyl, nitrophenyl, aminophenylsas 4-diethylaminophenyl, 4-dimethylaminophenyl and the like, naphthyl;and vinyl substituted aryl groups such as styryl, methoxystyryl,diethoxystyryl, dimethylaminostyryl,lbutyl-4-p-dimethylaminophenyl-l,3-butadienyl, 8-ethyl-4-dimethylaminostyryl, and the like; and where X is a sulfur, oxygen orselenium atom, and Z- is an anionic function, including such anions asperchlorate, fluoroborate, iodide, chloride, bromide, sulfate,periodate, p-toluenesulfonate, and the like. In addition, the pair R andR as well as the pair R and R can together he the necessary atoms tocomplete an aryl ring fused to the pyrylium nucleus. Typical members ofsuch pyrylium dyes are listed in Table 1.

atoms necessary to form a cyclic hydrocarbon radical includingcycloalkanes such as cyclohexyl and polycycloalkanes such as norbornyl,the total number of carbon atoms in R and R being up to 19;

R and R7 can each be hydrogen, an alkyl radical of from 1 to 5 carbonatoms or a halogen such as chloro, bromo, iodo, etc. and

R is a divalent radical selected from the following:

0 ll ll ll -c-o-, o-o-cm Preferred polymers useful in the present methodof forming aggregate crystals are hydrophobic carbonate polymerscomprised of the following recurring unit:

Each R is a phenylene radical including halo-substituted phenyleneradicals and alkyl-substituted phenylene radicals; and R and R are asdescribed above. Such compositions are disclosed, for example, in U.S.Pat. Nos. 3,028,365 by Schnell et al., issued Apr. 3, 1962 and 3,317,466by Caldwell et al., issued May 2, 1967. Preferably polycarbonatescontaining an alkylidene diarylene moiety in the recurring unit such asthose prepared with Bisphenol A and including polymeric products ofester exchange between diphenylcarbonate and2,2-bis(4-hydroxyphenyl)propane are useful in the practice of thisinvention. Such compositions are disclosed in the following U.S.patents: 2,999,750 by Miller et al., issued Sept. 12, 1961; 3,038,874 byLaakso et 211., issued June 12, 1962; 3,038,879 by Laakso et a1., issuedJune 12, 1962; 3,038,880 by Laakso et al., issued June 12, 1962;3,106,544 by Laakso et 211., issued Oct. 8, 1963; 3,106,545 by Laakso etal., issued Oct. 8, 196-3; and 3,106,546 by Laakso et al., issued Oct.8, 1963. A wide range of filmforming polycarbonate resins are useful,with completely satisfactory results being obtained when usingcommercial polymeric materials which are characterized by an inherentviscosity of about 0.5 to about 1.8.

The following polymers are included among the materials useful in thepractice of this invention:

dimethylcarb onate) Poly (3, 3-ethylenedioxyphenylene thiocarbonate).P013161, 4)-isopropylidenediphenylene carbonnte-cotereph t a ate 4 Poly(4, 4-isopropylidencdiphenylene carbonate).

Poly(4=, -t-isopropyledc ne diphenylene thioearbonate) Pcly(2,2-butancbis-4-phenylene carbonate). Poly (4. 4-lsopropylidcnediphenylenecarbonate-blocketltylene oxide) 8 Poly (4, t J-isopropylidenediphenylenecarbonate-blocktetramethylenoxide) 9 Polyl l,l-isopropylidenebis(Z-methylphenylene)-carbonate 10 Poly (4,4-is0propylidene diphenylene-co-l, l-phenylene carbonate 11 Poly (4,at-isopropylidenediphenylene-co-l, 3-phenylene carbonate).

12 Poly (4, 4J-isopropylidene dlpheuylene-co-4, 4-diphenylenecarbonate).

13 Poly(4, a-isopropylidenedlphenylene-co-e, 4'-oxydtphenylene carbon l4Poly(4, 4' 15opropylidenediphenylene-c0-4, 4-carbonyldiphenylenecarbonate).

15 Poly (4, 4-isopropylldenediphenylene-co-, 4-ethylene diphenylenecarbonate).

16. Polyl l, 4-methylenebis (Z-methylphenylene) carbonate].

17 Polyll, 1-](p-bromophenylethane)bis(4-phenylcne)carbonate 18 Polyll,a-iscpropylidenediphenylene-co-sul[onylbis-(4- phenylene)carbonate].

l9 Polyfl, 1-cyclohcxanebis( rphenylenekarbonatel.

20 loly[4, l-isopropylidcne bis(Z-chloropheuylcne)-carbonate].

- PolylZ, 2-(3-methylbutane)bls-4-phenylene carbonate].

olyl2, 2-(3, 3dimethylbutane) bisA-phenylene carbonate]. Poly {1,1-[1(1-naphthyl)]bis-4-phenylene carbonate 2 27 Poly[2, 2(4-methylpentane)bis-4-phenylene carbonate}.

2S. Pol y [4, 4 -(2-n0rbornylidene) diphenylene carbonate].

29 Polyi t, 4-(hexahydro'4, 7-methanolndan-5-ylldene)- diphenylenecarbonate].

30 Poly(4, 4-isopropylidenediphenylenecarbonate-blockoxytetramethylane).

The aggregate crystals formed according to the present invention canreadily be used for enhancing the sensitivity and extending the spectralrange of sensitivity of a variety of organic photoconductors andinorganic photoconductors including both nand p-type photoconductors. Atypical example of an inorganic photoconductor would be zinc oxide. Thepresent invention can be used in connection with many organic, includingorgano-metallic, photoconducting materials which have little orsubstantially no persistence of photoconductivity. Representativeorganometallic compounds are the organic derivatives of Group Illa, Na,and Va metals such as those having at least one amino-aryl groupattached to the metal atom. Exemplary organo-metallic compounds are thetriphenyl-p-dialkylaminophenyl derivatives of silicon, germanium, tinand lead, the tri-p-dialkylaminophenyl derivatives of arsenic, antimony,phosphorus, bismuth, boron, aluminum, gallium, thallium and indium.Useful photoconductors of this type are described in copending Goldmanand Johnson U.S. patent application Ser. No. 650,664, filed July 3, 1967and Johnson application Ser. No. 755,711, filed Aug. 27, 1968 now U.S.Pat. No. 3,607,257.

An especially useful class of organic photoconductors is referred toherein as organic amine photoconductors. Such organic photocondnctorshave as a common structural feature at least one amino group. Usefulorganic photoconductors which can be spectrally sensitized in accordancewith this invention include, therefore, arylamine compounds comprising(1) diarylamines such as diphenylamine, dinaphthylamine, N,N'diphenylbenzidine, N- phenyl 1 naphthylamine, N phenyl 2 naphthylamine,N,N' diphenyl p phenylenediamine, 2 carboxy 5 chloro 4methoxydiphenylamine, p anilinophenol, N,N' di 2 naphthyl pphenylenediamine, those described in Fox U.S. Pat. 3,240,597, issuedMar. 15, 1966, and the like, and (2) triarylarnines including (a)nonpolymeric triarylamines, such as triphenylamine, N,N,N',N'tetraphenyl m phenylenediamine, 4-acetyltriphenylamine, 4hexanoyltriphenylamine, 4 lauroyltriphenylamine, 4 hexyltriphenylamine,4 dodecyltriphenylamine, 4,4 bis(diphenylarnino)benzil, 4,4 bis(diphenylamino)bcnzophenone and the like, and (b) polymerictriarylamines such as poly[N,4" (N,N,N- triphenylbenzidine)],polyadipyltriphenylamine, polysebacyltriphenylamine,polydecamethylenetriphcnylamine, poly N (4 vinylphenyl)diphenylamine,poly N- (vinylphenyl) 0:,0c' dinaphthylamine and the like. Other usefulamine-type photoconductors are disclosed in U.S. Pat. 3,180,730, issuedApr. 27, 1965.

Useful photoconductive substances capable of being sensitized inaccordance with this invention are disclosed in Fox U.S. Pat. 3,265,496,issued Aug. 9, 1966, and include those represented by the followinggeneral forwherein T represents a mononuclear or polynuclear divalentaromatic radical, either fused or linear (cg, phenyl, naphthyl,biphenyl, binaphthyl, etc.), or a substituted divalent aromatic radicalof these types wherein said substituent can comprise a member such as anacyl group having from 1 to about 6 carbon atoms (e.g., acetyl,propionyl, butyryl, etc.), an alkyl group having from 1 to about 6carbon atoms (e.g., methyl, ethyl, propyl, butyl, etc.), an alkoxy grouphaving from 1 to about 6 carbon atoms (e.g., methoxy, ethoxy, propoxy,pentoxy, etc.), or a nitro group; M represents a mononuclear orpolynuclear monovalent aromatic radical, either fused or linear (e.g.,phenyl, naphthyl, biphenyl, etc.), or a substituted monovalent aromaticradical wherein said substituent can comprise a member, such as an acylgroup having from 1 to about 6 carbon atoms (e.g., acetyl, propionyl,butyryl, etc.), an alkyl group having from 1 to about 6 carbon atoms(e.g., methyl, ethyl, propyl, butyl, etc.), an alkoxy group having from1 to about 6 carbon atoms (e.g., methoxy, propoxy, pentoxy, etc.), or anitro group; Q can represent a hydrogen atom, a halogen atom, or anaromatic amino group, such as MNH-; b represents an integer from 1 toabout 12; and R represents a hydrogen atom, a mononuclear or polynucleararomatic radical, either fused or linear (e.g., phenyl, naphthyl,biphenyl, etc.), a substituted aromatic radical wherein said substituentcomprises an alkyl group, an alkoxy group, an acyl group, or a nitrogroup, or a poly(4'-vinyl henyl) group which is bonded to the nitrogenatom by a carbon atom of the phenyl group.

Polyarylalkane photoconductors are particularly useful in producing thepresent invention. Such photoconductors are described in US. Pat.3,274,000 by Noe et al., issued Sept. 20, 1966, French Pat. 1,383,461and in copending application of Seus and Goldman, titled PhotoconductiveElements Containing Organic Photoconductors, Ser. No. 627,857, filedApr. 3, 1967, now US. Pat. No. 3,542,544. These photoconductors includeleuco bases of diaryl or triaryl methane dye salts, 1,1,l-triarylalkaneswherein the alkane moiety has at least two carbon atoms andtetraarylmethanes, there being substituted an amine group on at leastone of the aryl groups attached to the alkane and methane moieties ofthe latter two classes of photoconductors which are non-leuco basematerials.

Preferred polyarylalkane photoconductors can be represented by theformula:

wherein each of D, E and G is an aryl group and J is a hydrogen atom, analkyl group, or an aryl group, at least one of D, E and G containing anamino substituent. The aryl groups attached to the central carbon atomare preferabl; phenyl groups, although naphthyl groups can also be used.Such aryl groups can contain such substituents as alkyl and alkoxytypically having 1 to 8 carbon atoms, hydroxy, halogen, etc., in theortho, meta or para positions, ortho-substituted phenyl being preferred.The aryl groups can also be joined together or cyclized to form afluorene moiety, for example. The amino substituent can be representedby the formula:

wherein each L can be an alkyl group typically having 1 to 8 carbonatoms, a hydrogen atom, an aryl group, or together the necessary atomsto form a heterocyclic amino group typically having to 6 atoms in thering such as morpholino, piperidino, tetrahydropyrrole, etc. At leastone of D, E, and G is preferably p-dialkylaminophenyl group. When I isan alkyl group, such an alkyl group more generally has 1 to 7 carbonatoms.

Representative useful polyarylalkane photoconductors include thecompounds listed in Table 3.

TABLE 3 Compound Number Name of Compound 1 4,4-benzylidenebis(N,N-diethyl-rn-toluidlne). 5 24,4-diamlno4-dimethylamino-2',2-dimethyltriphenylme ane. 3 4 ,4 -bls(diethylamlno) -2,6-diehloro-2 ,2" dimethyltrlphenylmethane. 44',4-bis(diethylamlno)-2,2-dlmethyldiphenylnaphthylmethane. 52,2"-ifimethyl-4,4,4"-trls(dimethylamlno)triphenylme ane. 1O 64,4-bis(diethylamtno)-4-dimethylamino-2,2-dimethyltrlphenylmethane. 7 4,4 -bis (diethylamino) -2-ehloro-2 ,2 -dlmethyl-4-dimethylaminotrlphenylmethane. 8 4' ,4 -bis(diethylamino)-4-dlmethylamlno-2,2 ,2

trimethyltriphenylmethane. 94,4-bis(dimethylamino)-?rchloro-2',2-dlmethyltriphenylmethane.

l0 4,4=-bis(dimethylamino)-2,2-dlmethyl-4-methoxytriphenylmethane. 11Bis(4-diethylamino)-1,1,1-triphenylethane. 12Bis(4-diethylamino)tetraphenylmethane. 13 4'Ags(benzylethylamino)-2,2"-dtmethyltriphenylme ane. 2O4,4-bls(diethylamlno)-2,2-diethoxytrlphenylmethane 154-bis(dimethylamlno)-1,l,1-triphenylethane.

1-(4-N,N-dimethylaminophenyl)-1,14iipheny1etl1ane.4dimethylaminotetraphenylmethane. 18 4-diethylaminotetraphenylmethane.

Another class of photoconductors useful in this invention are the4-diarylamino-substituted chalcones. Typical compounds of this type arelow molecular weight nonpolymeric ketones having the general formula:

wherein R and R are each phenyl radicals including substituted phenylradicals and particularly when R is a phenyl radical having the formula:

where R, and R are each aryl radicals, aliphatic residues of l to 12carbon atoms such as alkyl radicals preferably having 1 to 4 carbonatoms or hydrogen. Particularly advantageous results are obtained when Ris a phenyl radical including substituted phenyl radicals and where R isdiphenylaminophenyl, dimethylaminophenyl or phenyl.

Other photoconductors which can be used with the present aggregatecompositions include rhodamine B, malachite green, crystal violet,phenosafranine, cadmium sulfide, cadmium selenide, parachloronil,benzil, trinitrofluorenone, tetranitrofluorenone, etc.

In preparing photoconductive compositions in accordance with thisinvention, useful results are obtained when an organic, includingorgano-metallic, photoconductor is present in an amount equal to atleast about /z% by weight of the total solids added to the coatingsolvent. The upper limit of the amount of photoconductor pres ent can bevaried widely with up to 99% by weight of total solids being useful. Apreferred weight range for the photoconductor is from about 10 to aboutweight percent. Of course, if it is desired to use the present aggregatecompositions alone as the photoconductive substance, then nophotoconductor would be added. In addition to the photoconductorsdescribed above, polymeric photoconductors, e.g.,poly(N-vinylcarbazole), halogenated poly(N-vinylcarbazole) can also beused if desired.

According to the process of the invention, a pyrylium dye ashereinbefore defined is dissolved together with a hydrophobic polymer inan organic solvent system which contains a halogenated hydrocarbon.Suitable halogenated hydrocarbons can be selected from any solvent thathas substantial solvent action on both the dye and the polymer.Particularly useful solvents include, for example, chlorinated loweralkanes having from 1 to about 4 carbon atoms, e.g., dichloromethane,chloroform, carbon tetrachloride, 1,1 dichloroethane, 1,2dichloroethane, 1,1,2-trichloroethane, etc., and chlorinated aromatichydrocarbons, e.g., chlorobenzene, bromobenzene, 1,2-dichlorobenzene,etc. The conditions by which solution of the dye is elfected are notparticularly critical, providing that temperature is not a factor inadjusting the relative solubility of the dye and polymer in thesucceeding precipitation step. Recognizing this factor, the temperaturecan be any convenient temperature below the boiling point of the solventand below the decomposition temperature of any of the components.Similarly, sufficient agitation may be employed to promote solution in areasonable period of time.

The organic solvent system in which the dye and polymer are dissolvedmay contain additional solvents other than one chosen from thosementioned above. The reason for this will become evident uponconsideration of the succeeding steps in the process of the invention.

After the dye and polymer are dissolved in the halogenatedhydrocarbon-containing solvent system, a precipitating liquid is slowlycombined with the solvent system to cause the dissolved components ofthe solution to precipitate in a predetermined manner. Thisprecipitating liquid is characterized in that it is a non-halogenated,non-polar liquid in which neither the dye nor the polymer is soluble. Itmay be a liquid of low dielectric constant, for example, below about5.0. Suitable liquids include, for example, alkanes having from about 6to about 12 carbon atoms and which may be straight or branched chain,e.g., hexane, octane, decane, dodecane, 2,2,4-trimethylpentane, etc.,ligroin, and similar liquids and mixtures thereof.

The conditions of precipitation are such that a cocrystalline complex ofdye and polymer is formed in the presence of the organic solvent systemand the precipitating liquid. In order to ensure that a co-crystallinecomplex of the dye and polymer is formed and not merely a differentcrystalline form of dye or a precipitated form of polymer alone, theconditions of precipitation are carefully controlled. The dye andpolymer are initially dissolved in a good mutual solvent, such asdichloromethane. A precipitating liquid such as hexane, for example, isthen added to the solution. T his precipitating liquid is a non-solventfor both the polymer and the dye. The precipitate is examined todetermine whether it is dye or polymer alone. This can be done visuallyor by conventional analytical methods, and forms no part of the presentinvention. If it is determined that the dye has precipitated, forexample, there may be added to the solution a solvent, e.g., toluene,which is a better solvent for the dye than for the polymer. As analternative, the amount of dye in the solution initially may be reducedwith respect to the amount of polymer. As another alternative, thetemperature of the solution may be changed in such a direction that thesolubility of the dye increases at a greater rate than that of thepolymer. Conversely, if it is determined that the polymer hasprecipitated, the concentration of polymer may be reduced, apreferential solvent for the polymer may be added, or the temperaturemay be changed in such a direction as to increase the solubility of thepolymer with respect to that of the dye. The preferred method is toadjust the relative concentrations of dye and polymer so that theyprecipitate at about the same time and rate, in the form of theco-crystalline complex. As a rough guide to the relative amounts of dyeand polymer to be used initially, the solubility of each can bedetermined initially, and the concentration of each to be used selectedto be in approximately direct proportion to the solubility. For example,if the dye is less soluble, its concentration is reduced in the solventsystem. When it has been determined that the precipitate is theco-crystalline complex, the precipitating liquid is added in sufiicientquantity to completely precipitate the product. As previously indicated,the product can be identified as being the feature material of theaforementioned Light 12 application by virtue of the presence of aspectral absorption shift from that of the absorption spectrum of thedye alone. The shift appears as a change in the position of the spectralabsorption maximum from that of the dye by an amount of at least about10 nm.

It has been mentioned that the precipitating liquid is slowly combinedwith the solvent system to precipitate the desired co-crystallinecomplex. A preferred method of combining the two liquid media is to addone of the liquids dropwise to the other, at a rate of about one tothree drops per second. It is particularly preferred to dropwise add thesolvent system containing dye and polymer to the precipitating liquid,although the reverse will yield the same product, although at a muchslower rate.

The ratio of dye to polymer in the halogenated hydrocarbon-containingsolvent system can be varied over a limited range and still obtainreliable precipitation of the co-crystalline complex upon being combinedwith the precipitating liquid. Ratios of dye to polymer in the generalrange of from about 1:10 to about 2:1 and preferably 1:7 to 1:1generally yield reliable precipitation of the complex. Similarly, theratio of precipitating liquid to solvent can be varied over a relativelywide range, with values in the range of from about 50:1 to about 1:1being useful with 20:1 to about 2.5:1 being preferred.

After the formation of the feature material, it is separated from theliquid medium in which it is formed. Separation can be effected by anyknown means of separating a liquid from a suspended solid, such as byfiltration, centrifuging, decanting, and the like. A preferred method isto pour the contents of the vessel into a Biichner funnel fitted with apiece of filter paper. After removal of the liquid by, for example,application of a mild vacuum, the crystals are found to be retained bythe paper. The residual liquid is then removed from the crystals bydrying the filter paper and its contents at a suitable temperature offrom about 40 C. to about C. for a period of time ranging from about afew minutes to an hour, depending on the temperature employed.

After the crystals have been separated from the solvent system andprecipitating liquid, they are ready to be dispersed in a suitablepolymeric vehicle or binder system, as indicated previously. The binderis dissolved in a suitable solvent, and the feature material oraggregate crystals dispersed therein by any convenient technique such aslow or medium speed mixers of the types well known for the purpose. Ifnecessary or desirable, one or more photoconductors and any otherdesired addenda can also be added to the solution of vehicle containingdispersed aggregate crystals. It should be noted, however, that theaggregate crystals retain their crystalline form when so dispersed, andare not re-dissolved.

Solvents of choice for preparing the photoconductive compositions andelements to be coated therefrom in accordance with this invention caninclude a number of organic solvents for the polymeric vehicle such asalkanes having from about 5 to about 12 carbon atoms includingcycloalkanes such as pentane, cyclohexane; iso-octane, nonane, decane,dodecane, etc.; aromatic hydrocarbons including lower al-kyl-substitutedsuch as benzene, toluene, xylene, ethylbenzene, propylbenzene, etc.;ketones such as dialkyl ketones having 1 to about 3 carbon atoms in thealkyl moiety such as dimethylketone, methylethylketone, etc.; ormixtures of solvents. It is necessary, of course, that the solvent havesubstantially no solvent effect on the aggregate crystalline phase onceit has formed.

Preferred binders for use in preparing the photoconductive layers whichcan be formed in accordance with the method of this invention comprisepolymers having fairly high dielectric strength which are goodelectrically insulating film-forming vehicles. Materials of this typecomprise styrene-butadiene copolymers; polyvinyltoluenestyrenecopolymers; silicone resins; styrene-alkyd resins; silicone alkydresins; soya-alkyd resins; poly(vinyl chloride); poly(vinylidenechloride); vinylidene chlorideacrylonitrile copolymer; vinylidenechloride-vinylchloride copolymers; poly(vinyl acetate); vinylacetate-vinyl chloride copolymers; poly(vinyl acetals), such aspoly(vinyl butyral); polyacrylic and methacrylic esters, such as poly-(methylmethacrylate), poly(n-butylmethacrylate), poly- (isobutylmethacrylate), etc.; polystyrene, nitrated polystyrene;polymethylstyrene; isobutylene polymers; polyesters, such aspoly(ethylene alkaryloxyalkylene terephthalate); cellulose esters;phenol-formaldehyde resins; ketone resins; polyamides; polycarbonates;polythiocarbonates; poly(ethyleneglycol-co-bishydroxyethoxyphenylpropaneterephthalate); nuclear substituted polyvinyl haloacrylates; etc.Methods of making resins of this type have been described in the priorart, for example, styrenealkyd resins can be prepared according to themethod described in US. Pats. 2,361,019 by Gerhart, issued Oct. 24, 1944and 2,258,423 by Rust, issued Oct. 7, 1941. Suitable resins of the typecontemplated for use in the photoconductive layers of the invention aresold under such tradenames as Vitel PE-l, Cymac, Piccopale 100, SaranF-220 and Lexan 105 and 145. Other types of binders which can be used inthe photoconductive layers of the invention include such materials asparaifin, mineral waxes, etc.

Suitable supporting materials for coating sensitiZer-containingphotoconductive layers in accordance with the method of this inventioncan include any of a wide variety of electrically conducting supports,for example, paper (at a relative humidity above 20 percent);aluminumpaper laminates; metal foils such as aluminum foil, zinc foil,etc; metal plates, such as aluminum, copper, zinc, brass and galvanizedplates; vapor deposited metal layers such as silver, nickel, aluminumand the like coated on paper or conventional photographic film basessuch as cellulose acetate, polystyrene, etc. Such conducting materialsas nickel can be vacuum deposited on transparent film supports insufiiciently thin layers to allow electrophotographic elements preparedtherewith to be exposed from either side of such elements. An especiallyuseful conducting support can be prepared by coating a support materialsuch as poly(ethylene terephthalate) with a conducting layer containinga semiconductor dispersed in a resin or vacuum deposited on the support.Such conducting layers both with and without insulating barrier layersare described in US. Pat. 3,245,833 by Trevoy, issued Apr. 12, 1966.Likewise, a suitable conducting coating can be prepared from the sodiumsalt of a carboxyester lactone of maleic anhydride and a vinyl acetatepolymer. Such kinds of conducting layers and methods for their optimumpreparation and use are disclosed in U.S. 3,007,901 by Minsk, issuedNov. 7, 1961 and 3,262,- 807 by Sterman et al., issued July 26, 1966.

Coating thickness of the photoconductive composition on the support canvary widely. Normally, a coating in the range of about 10 microns toabout 300 microns before drying is useful for the practice of thisinvention. The preferred range of coating thickness is found to be inthe range from about 50 microns to about 150 microns before drying,although useful results can be obtained outside of this range. Theresultant dry thickness of the coating is preferably between about 2microns and about 50 microns, although useful results can be obtainedwith a dry coating thickness between about 1 and about 200 IIJJCI'OBS.

After the photoconductive elements prepared according to the method ofthis invention have been dried, they can be employed in any of thewell-known electrophotographic processes which require photoconductivelayers. One such process is the xerographic process. In a process ofthis type, an electrophotographic element is held in the dark and givena blanket electrostatic charge by placing it under a corona discharge.This uniform charge is retained by the layer because of the substantialdark insulating property of the layer, i.e., the low conductivity of thelayer in the dark. The electrostatic charge formed on the surface of thephotoconductive layer is then selectively dissipated from the surface ofthe layer by imagewise exposure to light by means of a conventionalexposure operation such as, for example, by a contact printingtechnique, or by lens projection of an image, and the like, to therebyform a latent electrostatic image in the photoconductive layer. Exposingthe surface in this manner forms a pattern of electrostatic charge byvirtue of the fact that light energy striking the photoconductor causesthe electrostatic charge in the light struck areas to be conducted awayfrom the surface in proportion to the intensity of the illumination in aparticular area.

The charge pattern produced by exposure is then developed or transferredto another surface and developed there, i.e., either the charge oruncharged areas rendered visible, by treatment with a medium comprisingelectrostatically responsive particles having optical density. Thedeveloping electrostatically responsive particles can be in the form ofa dust, i.e., powder, or a pigment in a resinous carrier, i.e., toner. Aprefererd method of applying such toner to a latent electrostatic imagefor solid area development is by the use of a magnetic brush. Methods offorming and using a magnetic brush toner applicator are described in thefollowing US. Pats: 2,786,439 by Young issued Mar. 26, 1957; 2,786,440by Giaimo, issued Mar. 26, 1957; 2,786,441 by Young, issued Mar. 26,1957; 2,874,063 by Greig, issued Feb. 17, 1959. Liquid development ofthe latent electrostatic image may also be used. In liquid development,the developing particles are carried as to the image-bearing surface inan electrically insulating liquid carrier. Methods of development ofthis type are widely known and have been described in the patentliterature, for example US. Pat. 2,907,674 by Metcalfe et al., issuedOct. 6, 1959. In dry development processes, the most widely used methodof obtaining a permanent record is achieved by selecting a developingparticle which has as one of its components a low melting resin. Heatingthe powder image then causes the resin to melt or fuse into or on theelement. The powder is, therefore, caused to adhere permanently to thesurface of the photoconduct layer. In other cases a transfer of theelectrostatic charge image formed on the photoconductive layer can bemade to a second support such as paper which would then become the finalprint after development and fusing. Techniques of the type indicated arewell known in the art and have been described in the literature such asin RCA Review vol. 15 (1954), pp. 469-484.

The following examples are included for a further understanding of theinvention.

EXAMPLE 1 A 0.2 gram portion of the dye4-(4-dimethylaminophenyl)-2,G-diphenylthiapyrylium perchlorate and a 0.2gram portion of poly(4,4'-isopropylidenediphenylene carbonate) aredissolved with stirring in 10.0 grams of dichloromethane. To thesolution of dye and polymer are added 5.0 grams of toluene, causingpartial precipitation of the dye. Sufficient dichloromethane is added toredissolve the dye, or about 5 grams. To the solvent system comprisingdichloromethane and toluene are added about 50 grams of n-hexane, withstirring. After a few minutes, a bluish-green crystalline materialprecipitates from the mixture of the solvent system and precipitatingsolvent. The crystals are separated from the solvent system by pouringthe contents into a Buchner funnel containing filter paper and applyinga mild vacuum to the funnel using an aspirator. The crystals thusseparated are then dried by heating them to about C. for about 15minutes. They are identified as being aggregate crystals by virtue ofthe fact that their spectral absorption maximum is at a wavelength of685 nm., while that of the dye alone is 590 nm. Photoconductivecompositions are then prepared by dissolving 0.58 gram of a modifiedpoly(vinyl butyral) containing approximately '9 Weight percent of poly(vinyl alcohol) units (Butvar B-76, Monsanto Chem- 15 ical Co.) and 0.40gram of 4,4'-benzylidenebis(N,N-diethyl-m-toluidine) in 9.0 grams oftoluene, and adding to each of three such solutions a separate portionof aggregate crystals prepared as described above. The weight ofcrystals added to the first solution is 0.04 gram, to the secondsolution, 0.08 gram, and to the third solution, 0.12 gram. Eachcomposition thus obtained is shaken in a metal container with smallsteel balls for about minutes to produce a uniform mixture. Each of thecompositions is then coated at a wet thickness of 100 microns on aconductive support comprising poly(ethylene terephthalate) film supportbearing a layer of cuprous iodide as in the aforementioned Trevoy U .8.Pat. 3,245,833, and dried. The resultant electrophotographic elementsare then electrostatically charged under a negative corona source untilthe surface potential, as measured by an electrometer probe, reachesabout 600 volts. The charged element is then exposed to a 3000 K.tungsten light source through a stepped density gray scale. The exposurecauses reduction of the surface potential of the element under each stepof the gray scale from its initial potential, V0, to some lowerpotential, V, whose exact value depends on the actual amount of exposurein meter-candleseconds received by the area. The results of thesemeasurements are then plotted on a graph of surface potential V vs. logexposure for each step. The actual negative speed of the photoconductivecomposition can then be expressed in terms of the reciprocal of theexposure required to reduce the surface potential to any fixedarbitrarily selected value. Herein, unless otherwise stated, the actualnegative speed is the numerical expression of 104 divided by theexposure in meter-candle-seconds required to reduce the 600 volt chargedsurface potential by 100 volts. The speeds of the elements thusobtained, in order of increasing concentration of co-crystalline complexcrystals, are 1600, 3200 and 4000. As a control, a composition isprepared as first described above for preparing the co-crystallinecomplex but omitting the polycarbonate. The precipitate thus obtainedis, therefore, only dye. Dye crystals are incorporated in aphotoconductor-containing composition and coated to formelectrophotographic elements as above. The speeds of the elements thusobtained are less than 20. It is thus seen that the crystals formedaccording to the process of this invention truly comprise aco-crystalline complex of dye and polymer of the type described byLight.

EXAMPLE 2 The crystals of co-crystalline complex formed as in Example 1are incorporated in a photoconductive composition similar to that ofExample 1 except that a polystyrene resin having a ditferent averagemolecular weight (Koppers 8X, Koppers Co., Inc.) is used as the binderfor the photoconductive composition and cyclohexane as the coatingsolvent. The weight of aggregate or co-crystalline complex crystals usedis 0.12 gram. Coating conditions are the same as in Example 1. The speedof the element thus produced is 800, while the speed of an elementprepared from dye alone instead of co-crystalline complex is zero. Thisis because the dye alone does not sensitize the photoconductor in apolystyrene binder system.

EXAMPLE 3 The general procedure of Example 1 is followed in preparingco-crystalline complex crystals. In the present example, a 2.5 Wt./vol.percent solution of the dye in dichloromethane is prepared to which isadded a 2.5 wt./ vol. percent solution of the resin, also indichloromethane. The dye and resin used are those of Example 1. Nofurther solvent is added to the dye-polymer solution. To the solution isthen added dropwise an amount of ligroin having a boiling range of about63 to 75 C. (Eastman Organic Chemicals P513), the amount being about 2.5times the volume of solvent used to dissolve dye and polymer.Precipitation of co-crystalline complex occurs as in Example 1, afterwhich the crystals are isolated and dried.

16 The crystals are then used to prepare three elements as in Example 1.Theseelements each give an electrophotographic response similar to thatobtained in Example 1. The crystals are identified as the complex by thepresence of the shift in the wavelength of maximum spectral absorption.

EXAMPLE 4 The procedure of Example 3 is followed using 2.0 wt./ col.percent solutions of dye and polymer, and 1,2-dichl0- roethane as thesolvent. The precipitating liquid is n-hexane (Eastman Organic ChemicalsP1135), the amount being about 2.4 times the volume of solvent used.Crystals similar to those in Example 3 are obtained which show thespectral absorption shift characteristic of the aggregate orco-crystalline complex and which confer similar high speeds to elementsprepared therefrom.

EXAMPLE 5 The procedure of Example 3 is followed using twice the weightconcentration of polymer in the initial solution. The percipitatingliquid is the liquid of Example 4, the amount being 2.4 times the volumeof solvent. Results simular to those in Examples 3 and 4 are obtained.

EXAMPLE 6 Using the dye and polymer of Example 1, a dye-polymer solutionis prepared which contains 2.0 weight percent of both dye and polymer indichloromethane. The order of addition of Example 1 is reversed, in thatthe dyepolymer solution is added dropwise to the precipitating liquid.The precipitating liquid is the ligroin of Example 3, its volume beingthree times that of initial dye-polymer solution. Crystals of aggregatebegin to form immediately upon addition of the solution to theprecipitating liquid. When the entire volume of initial solution hasbeen added, the crystals are separated and dried as in the previousexamples. The method of this example is preferred, as the completeprecipitation requires a much shorter time. Identification of thecrystals as the aggregate is carried out as before, as is thepreparation of an electrophotographic element which, when tested, hassimilar high speeds.

EXAMPLE 7 A solution is prepared from the following ingredients:

4 (4 dimethylaminophenyl) 2,6 diphenyl thiapyrylium fluoroborate (DyeII) g 2.7 4-(4 dimethylaminophenyl) 2 (4-eth0xyphenyl)- 6-phenylthiapyry-lium fluorobor-ate (Dye Ill) g 0.3 Lexan g 3.0 Methylenechloride ml 525 Toluene ml 375 gate crystals are then used to preparethe following compositions:

Amount Composi- Millition Ingredients liters Grams Chlorinatedpolyethylene (65.8% 01), Texas 1. 2

Eastman Co. 'IX199-14 (binder). A 4,4-diethylamino-2,2-dimethyltriphenyl0. 8

"""" methane (photoconductor).

Aggregate crystals 0. 2 Toluene 12 Polyvinyl-vinylidene chloride,(Goodrich 1. 2

Rubber 00., Glenn 222}. B 4,4-diethylam.ino-2,2-dimethyltriphenyl 0. 8

""'"' methane (photoconductor).

Aggregate crystals 0. 2 Toluene 12P01y(vinyl-m-bromobenzoate-co-viny1 1. 2

acetate. 0 4,4-diethylamlno-2,2-dimethyltriphenyl 0. 8

""""" methane (photoconductor).

Aggregate crystals 0. 2 Toluene 12 Porcelain balls A3") are added toeach of the above compositions and each is shaken for two hours on a lowamplitude, high frequency vibrating mixer. Each of the resultingformulations is coated at .004" wet thickness on a 0.4 CD. nickel coatedsubstrate to form an electrophotographic element which is measured forits speed as described above. The speeds are shown below.

SPEED Positive Negative omposition Shoulder Toe Shoulder Toe Similarresults are also obtained using Dye I and 4-(4-dimethylaminophenyD-Z-(4-ethoxyphenyl) 6 phenyl thiapyrylium perchlorate(Dye IV).

EXAMPLE 8 Two stock solutions are prepared each containing: Dye H g 0.44Dye III g 0.06 Lexan 145 g 3.2 Methylene chloride ..ml 350 Toluene ml250 One solution is added dropwise to 3000 ml. of hexane and cooled to-80 C. by means of a Dry Ice-acetone bath. The other solution is addedto 3000 ml. of hexane maintained at room temperature (-23 C.). Theresulting aggregrate precipitates are collected by suction filtration.Upon examination of the two precipitates, it is observed that the oneprepared at 80 C. has a much finer particle size than the one preparedat room temperature.

EXAMPLE 9 Four solutions are prepared from the ingredients listed below:

Solution No. 1 2 3 4 Lexan 145, g 3. 5 3. 5 1. 2 13. 3 Dye, H g 0.440.44 0.15 6.65 Dye g 0.06 0. 06 0. 02 Methylene chloride, ml 350 850 1181, 750 Toluene, ml 250 250 83 1, 250

EXAMPLE 10 A mixture is prepared of 0.9 gram of polyvinylcarbazole, 0.2gram of the aggregate crystals of Example 7 and 12 ml. of benzene bystirring the polyvinylcarbazole into solution, adding the aggregatecrystals and agitating with a vibrating shaker for 2 hours using A;-inch porcelain balls. The formulation is then coated at 27 C. on a 0.40D. nickel-coated substrate at a .00 wet thickness. A control coating isalso prepared which contains 1.8 grams of polyvinylcarbazole, 30 ml. ofbenzene, 0.2 g. of Lexan 145 and 0.18 g. of Dye II and 0.02 g. of DyeIII. The Lexan and dye of this composition are simply dissolved togetherand are present as a homogeneous combination not in the form of aco-crystalline complex. This control coating is also used to form anelectrophotographic element as above and the two elements are measuredfor their speed. The element containing the aggregate has a positive 100volt shoulder speed of 700 and a toe speed of 100. The control elementwhich contains no aggregate has a 100 volt shoulder speed more than 10times slower than the aggregate-containing element. The 100 volt toespeed of the control is 0.

The term co-crystalline complex as used herein has reference to acrystalline compound which contains dye and polymer moleculesco-crystallized in a single crystalline structure to form a regulararray of the molecules in a three dimensional pattern.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

We claim:

1. A method of preparing a heterogeneous photoconductive compositioncontaining a dispersion of a cocrystalline complex of dye and polymer,said method comprising the steps of:

(a) dissolving a pyrylium dye and a carbonate polymer in an organicsolvent system containing a halogenated hydrocarbon to form a solutionthereof, said dye and carbonate polymer having substantially equalsolubilities in the organic solvent system;

(b) slowly combining the solution with a precipitating liquid comprisedof a non-halogenated, non-polar organic liquid in which the dye andcarbonate polymer are insoluble, thereby precipitating a co-crystallinecomplex of dye and polymer;

(0) separating the complex from the solvent system and precipitatingliquid; and

(d) dispersing the complex in a film-forming organic polymeric vehicleto form a heterogeneous photoconductive composition.

2. A method according to claim 1 comprising the additional step ofdispersing an organic photoconductor in the organic polymeric vehicle.

3. A method according to claim 1 wherein the carbonate polymer containsthe following recurring unit:

wherein each R is a phenylene radical and R and R when taken separatelyare selected from the group consisting of a hydrogen atom, an alkylradical having from one to about 10 carbon atoms, and an aryl radical,and when taken together represent the number of carbon atoms necessaryto complete a cyclic hydrocarbon radical, the total number of carbonatoms in R and R being up to 19.

4. The method as described in claim 1 wherein the halogenatedhydrocarbon in said solvent system is selected from the group consistingof dic'hloromethane, 1,2-dichloroethane and 1,1,2-trichloroethane andsaid precipitating liquid is selected from hexane and octane.

'5. A method according to claim 1 wherein the dye is selected from thegroup consisting of pyrylium and thiapyrylium dye salts.

6. -A method according to claim 5 wherein the dye salt corresponds tothe formula:

wherein each of R and R is a phenyl radical, R is analkylamino-substituted phenyl radical having from 1 to about 6 carbonatoms in the alkyl moiety, X is selected from the group consisting of anoxygen atom and a sulfur atom, and Z- is an anionic function.

19 7. A method of preparing an electrophotographic element comprisingthe steps of:

(a) dissolving in an organic solvent system containing a halogenatedhydrocarbon, a pyrylium dye salt corresponding to the formula:

R; i R2 wherein each of R and R is a phenyl radical, R is analkylamino-substituted phenyl radical having from 1 to about 6 carbonatoms in the alkyl moiety, X is selected from the group consisting of anoxygen atom and a sulfur atom, and Z- is an anionic function,

and a carbonate polymer having the recurring unit:

wherein each R is a phenylene radical and R and R when taken separatelyare selected from the group consisting of a hydrogen atom, an alkylradical having from one to about 10 carbon atoms, and an aryl radical,and when taken together, represent the number of carbon atoms necessaryto complete a cyclic hydrocarbon radical, the total number of carbonatoms in R and R being up to 19';

(b) adding a precipitating liquid comprised of a nonhalogenated,non-polar organic liquid in which the dye and carbonate polymer areinsoluble, said liquid having a dielectric constant less than about 5;

(c) precipitating a co-crystalline complex of pyrylium dye and carbonatepolymer;

(d) separating the complex from the solvent system and precipitatingliquid;

(e) dispersing the complex and an organic photocondoctor in a polymericfilm-forming organic vehicle; and

(f) coating a thin film of the resultant dispersion on a conductivesupport.

8. The method as described in claim 7 wherein said dye salt is selectedfrom the group consisting of salts of 4(4-dimethylaminopheny-l)-2-(4-ethoxyphenyl)-6-phenylthiapyrylium,4-(4-dirnethylaminophcnyD-Z,6-diphenylthiapyryliurn and4-(4-dimethylaminophenyl)-2-(4-methoxy- 15phenyl)-6-phenylthiapyryliurn.

9. The method as described in claim 7 wherein said polymer ispoly(4,4'-isopropylidenediphenylene carbon- GEORGE F. LESMES, PrimaryExaminer I. R. MILLER, Assistant Examiner U.S. C1. X.R,

961; 252-50l; 260-342, 37 PC

